Concerning the system for controlling automatically the gain for correlation spikes in the communication device, there is known e.g. that disclosed in U.S. Pat. No. 4,899,364. FIG. 7 indicates the fundamental construction of the prior art system described above.
In the figure, a correlation spike outputted by a demodulator 33 is compared with either one of two threshold voltages, positive or negative, given by a reference voltage generating section RV in a comparator section CA, depending on the polarity thereof. Responding to a result thus obtained, a counter control section CC selects a first or a second clock in a clock selecting section CL to give it to a counter section CU and makes it count in a positive or negative direction. A current responding to a count value thus obtained is generated by a gain control current generating section GC. The gain of a variable gain amplifier 32 disposed on the output side of a correlator 31 is controlled by this current so that the correlation spike is comprised between the positive and the negative threshold voltage.
However, by this system, a circuit for detecting a peak value is necessary for AGC and further a peak value detecting circuit is required for a peak value of each of a positive and a negative correlation spike signal. In addition, since the variable gain amplifier 32 is controlled, depending on the number of correlation spikes, it should be controlled logarithmically, depending on characteristics thereof, and for this reason, the gain control current generating section GC should have logarithmic characteristics.
Consequently the prior art system has a drawback that the circuit scale is great for this reason, which raises fabrication cost.